Nuclear or gamma cameras are often used to measure gamma radiation emitted by a body under examination. By measuring the energy level and the location of the gamma ray emissions, an image representative of the gamma radiation emitted from the body under examination can be created.
One application of nuclear or gamma cameras is in medical imaging, where one or more radionuclides are introduced into a region of interest within a patient. These radionuclides decay, thereby emitting gamma radiation characterized by photons having one or more characteristic energies. For example, Tc-99m emits photons having a photopeak located at approximately 140.5 keV.
In practice, however, the radiation spectrum resulting from the decay of a radionuclide is spread over a range of energies. Compton interactions with electrons in the gamma camera scintillation crystal and the body being imaged contribute to this spread. Photons which experience Compton interactions are deflected in angle and experience an energy loss compared to primary (i.e. non-scattered) photons, but can be detected along with primary photons, thereby resulting in spurious scatter counts at energy levels below a primary photopeak.
Variations in detected energy levels can also be caused by the energy resolution of the measurement equipment itself. Thus, depending on its energy resolution, a gamma camera exposed to monochromatic gamma radiation will produce output counts over a range of energies as illustrated in FIG. 1. One measure of a gamma camera's energy resolution is its full width half maximum ("FWHM"), which is defined as the full width of the energy distribution at half the maximum amplitude of the peak, assuming a monochromatice radiation input. The energy resolution and therefore the FWHM varies with camera manufacture and design as well as with photopeak energy. Thus, primary radiation which contributes to a useful image will as a practical matter be detected over a range of energies located in the region of a photopeak.
Various energy-based techniques for correcting for Compton scatter and measurement equipment variation have been attempted. One such technique utilizes a primary photopeak energy range along with a wide scatter energy range located below the primary photopeak in the Compton region. An estimate of the scatter counts falling within the photopeak energy range can then be generated based on the counts falling within the scatter energy range. It is, however, difficult to estimate the scatter counts using this method, one reason being that the scatter component falling within the photopeak energy range varies non-linearly with the scattering media. The relative width of the scatter energy range, together with the generally non-linear spatial distribution of counts within the scatter energy range, lead to further difficulties and concomitant errors in estimating the scatter component within the photopeak energy range.
In an alternate scheme, the photopeak region is divided into two abutting but non-overlapping energy ranges symmetrically located about the photopeak. A scatter fraction is estimated based on the count ratio between the upper and lower ranges. This method presents two major difficulties. First, it is difficult to determine a scatter relationship between the two ranges. Second, the method is sensitive to count spillover caused by energy window drifts that may occur during data acquisition. This method is thus relatively difficult to implement.
Yet another method is described in U.S. Pat. No. 5,371,672 to Motomura, et at. This method utilizes a plurality of energy ranges, the first being a photopeak range, while the second and third ranges are relatively narrow and abutting or overlapping each side of the photopeak range. In some cases, the upper or third range is omitted. According to this method, a separate event count is maintained for each energy range, each energy range thus being treated as a separate window for each of a multiplicity of x-y positions, thereby creating a separate image representation for each energy range. After being stored, the counts maintained within each window for a given x,y position are subsequently combined to estimate the scatter component. This technique has several disadvantages. By placing the scatter windows adjacent or overlapping with the main window, the second and third windows contain counts attributable to the main photopeak (i.e. non-scatter or primary counts), thereby reducing the accuracy of the scatter correction. This problem is exacerbated as the width of the main photopeak energy range is narrowed. The use of a separate window for each energy range also requires significant computer memory and real time processing.
Thus, it can be seen that a scatter correction technique which is accurate, simple to implement, and exhibits reduced memory requirements is needed.